Organic polymer-based electroluminescent devices (OLEDs) have the potential for providing inexpensive alternatives to alphanumeric displays and x-y addressable displays. A simple OLED may be constructed from an electroluminescent layer sandwiched between an electron injection electrode and a hole injection electrode. More complicated devices utilize electron and hole transport layers between the above mentioned electrodes and the electroluminescent layer. Devices based on poly (p-phenylenevinylene) (PPV), or derivatives thereof, have been demonstrated with sufficient quantum yields to be commercially attractive.
To make a pixelated OLED display, the transparent electrode (usually indium tin oxide, or ITO) is usually patterned into strips running in one direction. After deposition of the organic layers, the final electrode (usually the cathode) is patterned into strips in the orthogonal direction. The cathode is typically constructed from a reactive metal such as Ca or Mg. As a result, the use of standard positive resist techniques is difficult at best. These metals are highly reactive toward water, and also react (though less rapidly) with organic molecules possessing active hydrogens such as alcohols. Since standard photoresist is based on novolak (a phenolic polymer), there is already the potential for significant reaction when the polymer is cast on the metal. Furthermore, the development mechanism of positive resist requires water to be present in the polymer during exposure. This water reaches the metal, and the subsequent reaction degrades the electrode. This situation is further exacerbated during the actual development and rinse steps. If water comes in direct contact with the electrode metal, and if the adhesion of metal to electroluminescent polymer or resist is not of the highest quality, the electrode may be damaged by the undercutting effect as water penetrates into a region that should be protected by resist. Even if the electrode is not lost due to the interaction, some pinholes are likely to be present in the metal. These pinholes allow solvent (and water) from the resist to penetrate directly to the surface of the active organic material.
As a result of these difficulties, the primary patterning technique that has been used to date is a lift-off process. A photoresist mask is formed on the substrate before any active organic layer is deposited, and this mask is used to pattern the metal by interrupting its continuity. While the technique has been successfully used, it has a number of drawbacks. First, it requires either an undercut resist profile, which is more difficult to regularly fabricate, or else an oblique evaporation step for the metal. Second, it is more difficult to use with polymers since the polymer must now be cast over a resist profile that has a tendency to interrupt also the polymer deposition. The polymer may conformally cover the resist features, and then the resist can no longer serve its function of interrupting the metal film. These problems are greatly magnified in three-color displays.
Finally, the standard methods of polymer film coating uniformly cover the entire substrate. In multi-color displays, the polymer must be selectively removed from the pixel areas where the next polymer (emitting a different color) is to be placed. Lift-off processes do not lend themselves to such selective removal.
Techniques based on a conventional negative resist consisting of a photocrosslinkable rubber that does not require water for its action have been demonstrated. However, conventional cross-linking resists of this type are developed by swelling. Both the irradiated and unirradiated regions imbibe developer, but only the unirradiated region dissolves. As a result, the metal is exposed fully to the developer solution. The developer, consisting of 2-ethoxyethanol, penetrates any pinholes in the metal and reacts with the metal or with the active organic electroluminescent material leading to degradation of the electrode. Furthermore, the use of organic solvent for developing results in large quantities of more or less toxic liquid waste. The disposal cost further detracts from the use of this method on a production scale.
Broadly, it is the object of the present invention to provide an improved OLED.
It is a further object of the present invention to provide a photolithographic method for patterning the top electrode of OLEDs that is applicable to multi-color displays.
It is a still further object of the present invention to provide a photolithographic method that does not expose the cathode to water or active hydrogen.
It is a yet further object of the present invention to provide a photolithographic method that does not generate the toxic liquid waste discussed above.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.